Abstract: The crop water stress index (CWSI) based on the daily canopy temperature at 14 : 00, has been calculated on the basis of winter wheat field experiment. The results indicate that the CWSI with drying process higher than that with wet process at the same time; after 4~6 days of irrigation, CWSI drops to a minimum, it is a period of plant recovering from a stressed condition. And, the steady increase trend of CWSI between two times of irrigation shows that crop water stress increases with time in this period. The above inclusions show that the plant water stress is due to the lack of water. There is obvious negative correlation between CWSI and leaf water potential. When CWSI equals 0.4, it could be a critical index in indicating winter wheat water stress.